Testing of semiconductors



April 1954 J. R. HAYNES ETAL 06 TESTING OF SEMICONDUCTORS Filed Dec. 2, l950 2 Sheets-Sheet l ['76. FIG. 2

FIG. 3 W, FIG. 4

- J R. HAYNES J A. MORTON ATTORN V Patented Apr. 27, 1954 UNITED STATES ATENT OFFICE TESTING OF SEMICONDUCTORS Application December 2, 1950, Serial No. 198,864

9 Claims.

-This invention relates to the electrical testing of bodies of semiconductive material.

An object of the invention is to facilitate the manufacture of transistors and other semiconductor translating devices and, more particularly, to enable the manufacturer to determine readily whether or not, or to what extent, a given ingot, crystal or other body of semiconductive material, such as germanium or silicon, has electrical properties desired in such devices. A related object is to enable one to cull out unsuitable or inferior ingots or the like and thereby to avoid waste in the time-consuming and costly cutting and grinding operations involved in subdividing an ingot into the minute plates and filaments usually desired for translating devices.

The operation of various semiconductor translating devices is commonly explained in terms of mobile charge carriers, via, holes and excess electrons, that are created within the semiconductor at the place or places where the device is appropriately excited by an external energy source. The holes and electrons (which represent mobile charges of positive and negative sign, respectively,) tend, however, to recombine with each other and so to disappear in a relatively short time that is of the order of microseconds and that may be as much as hundreds of microseconds. The rate of recombination, or average lifetime, of such mobile charges is an important factor in the operation of semiconductor devices; it is determined in part by an intrinsic property of the semiconductive material itself, and in part by the proximity and character of the surface of the semiconductor.

A further and more specific object of the invention is to enable the comparison of semiconductor bodies with respect to electronor holelifetime, and the grading of such bodies with respect to the above-mentioned intrinsic property, 'viz., intrinsic lifetime.

The present invention contemplates the generation of mobile charge carriers at a first point in the semiconductor body to be tested under conditions such that the charge carriers are free to diffuse undisturbed through the body, efiicient detection of the Charge carriers at a second point separated from the first, and measurement of the effect of varying the distance between the two points on the density of charge carriers at the second point. The invention is featured, in part, by the use of a beam of radiant energy, and specifically an amplitude-modulated beam of light, to generate the charge carriers, and by movement of the beam to vary the distance over which the carriers diffuse before they reach the detectio point. Another feature of the invention resides in preparation of a surface of the semiconductive body to be tested so as to reduce the rate of recombination of holes and electrons at the surface, i. e., to increase the surface lifetime, whereby a readier and more accurate determination can be made of the rate of recombination within the mass at a distance from the surface.

The nature of the present invention and its various features and objectives will appear more fully upon consideration of the specific embodiments illustrated in the accompanying drawings and hereinafter to be described.

In the drawings:

Fig. 1 illustrates a typical ingot of semiconductive material with longitudinal axis and sectioning planes indicated thereon;

Figs. 2, 3 and 4 show diagrammatically an arrangement suitable for practice of the invention;

Fig. 5 illustrates an alternative arrangement; and

Fig. 6 comprises explanatory curve diagrams.

Referring more particularly now to Fig. 1, there is shown a typical ingot of germanium which may be, for specific example, three inches long and of roughly circular cross-section having an average diameter of about one-half to one inch.

: In the manufacture of translating devices from such an ingot the end portions a are ordinarily removed and discarded inasmuch as the properties of those portions are likely to dilfer widely from those of the bulk of the ingot. The ingot is then sawed into slices about 0.020 to 0.040 inch thick and these are in turn out into minute plates or filaments which are then commonly subjected to special surface treatment and assembled into translating devices. The devices on being tested, however, may be found to vary widely in performance, some or all of those derived from a particular ingot being perhaps inferior or completely unsatisfactory because of intrinsic properties of the material.

In the practice of the invention, it may be found that each of the ingots in any given run, i. e. those from a given melt, is fairly homogeneous with respect to intrinsic lifetime and that a single test of each will enable the ingots to be graded according to intrinsic lifetime. If the ingots are not homogeneous, each may be cut transversely of the axis as indicated by the section lines in Fig. 1, into several large blocks and these blocks then tested, for we find that the intrinsic lifetime .of such an ingot varies gradual- 3 1y both radially and longitudinally in the ingot. In manufacturing operations, too, it will ordinarily be of less importance to obtain an absolute measure of lifetime than to obtain a test result indicating the relative intrinsic lifetime of a particular ingot or part thereof or whether, in

respect of lifetimarthe particular test "part "meets a predetermined reference standard or falls withr in predetermined limits.

Figs. 2 to i illustrate a specific testing arrange-=- ment in accordance with the invention... 'Ihere.v is represented at 5 an ingot of germanium, or one" of the described larger subdivisions-thereof, .of. which one of the end faces,- 1, haszbeen :ground: flat, polished and treated ima -manner to'be dea scribed. At 8 is shown a collector electrode comprising a wire, or cat whisker, the end.of,-which-.

rests on the surface I and makes a rectifying point contact therewith at C in a manner well-e known in the art. This point contact may be electrically formed to increase its-sensitivity; ass a collector in thc-mannerset'forth in-;the applicae tion: oi -7W. G. Pfann, SerialNo. 68,596 filedrDee' cember-Sl, 1948 A base:electrode;9 makes a.loW.-- resistance contact with.the;lateralsurfacer of" thetest piece, and. an: electrical circuit.is.completed between thetwo electrodes through a bi asingbattery liland a.resistor H. ThecontactQ may be provided=bya: chuckor carriagei ll'in which the test piece is clamped as shown in figi 4'.

The battery it} is poledxin ;the.so-.called re-' verse direction, i. e.-; in thedirection-oppositeto that ofeasycurrent flowsthrough the point rec tifier contact. 4 x

It will be assumed for specificcexample that the germanium. iswof n-rtype, in which case the collectorelectro'de il'hasa-znegative bias asshownand the mobile charge-carries? ofa interestare holes; A-beam of-lightindicated at 13 and shaped by; a=-slit l511s directedperpendicularly; against.

thesurface l to illuminateanarrowstrip, [4 (Fig. 3) thereof that is disposed symmetrically with re? lation to: coilector contactpoint a. .small distance-r therefrom. lhezdistanceris-made ad- Under holesdiffuse in all directions sthrough. the body,

becoming less. and less:concentratedcatgreater distances from. the. illuminated: portion of the. surface.

acrossresistor ll, eveninthe; absence of; the. light beam, due to the steadybiasing; current that flows in thecollectorcircuit; This constant voltage is :excluded from consideration by modulating the light beam at low-frequency o1 ofthe order of 20 cycles per/second, .for example, as by means of the light-chopper' l8 in Fig. hand by employing air'alternate currentvoltmeter 20- or equivalent device responsive onlyto' the. ale ternating voltage across resistor=l i 1 The "invention purposes to iobtain'a measure of t the. intrinsic Thole-lifetime :of :the test 1 piece: from:

a, detrminatiorrof the manner in avhichtheid'emi the conditions. described the: light beamgenerates hole-electron.pairs-at. the surface 1- and to some small depthnintohthe body, and the:

Some of: the: holes-=reach- I electrode. 8 v and giverise to antincrement of-current: in-..the external circuit which can be-measured bylmeas-r uring the increment. of voltagev drop. across resistor- H. A'certainstead-y voltage'drop appears- 4 sit of holes varies with distance along the surface l' from their place of origin. It will be understood that in Fig. 4 the change in voltage drop across resistor H due to the arrival of holes generated by the light beam affords a measure of the hole density -at a fixed point, namely, the point .0 immediately underthe collector electrode, and that the variation in the distance -r 'of this point from the place of origin of the holes can b'eteffected by shifting the position of the light beamtowardor. away from the point C.

Aatheh'Qlesdifiuse, recombination is continucusly;-taking place-so that the holes are of limitedaveragelifetime 7p. The tendency of the 'surface:.'|'; howevenzis to accelerate recombinai the surface from the generating point greater than the irate: of t density 1 decrease other; directions through .thezbody Forathespurposesof thee presentinvention theesurface l is treated-toreduce substantially the tendency i the surface to accelerate:recombinationand to .eliminate it zas' nearly aspossible; If the-surfacea-l did not produce any, recombination. it: wouldrefiect holes back-into the body of;the--test piece -'just as-if the latter were infinite'in' extent, and thediffusion .pattern along, thesurfacer-would. beamore-simply and directly related toithe: intrinsic lifetime of the material. 'I'hepracticaleffect ofzsoireducing surface recombination istoincrease=the=accuracy of. measurement of intrinsic lifetime, to extend therange of conditionsrunder which the inven tion maybe; applied, audio-simplify; the. pro cedure' involved in--. obtaining satisfactory approximate measurements.-. 1

One 'sui-tabley-method of. treating the surface, involving. the'applicationof antimony oxychlo ride, is disclosed in application .ior-United States patentSerialNo. 175,648filed'Jul-y 24,- 15950, by. J. RtilaynesandrRr D.--.Heidenrich and also .in an article entitled The Mobility: and. Lifeiof Ine I jected Holes and .Electrons l-in :Germanium. by J:R.-.Haynes and W; Shockley, .81-Physical Re-- view-335, 8-38 (1951-) By this-anethod-recombinationvelocities as .=low as afew hundred-centimeters-pers'econd-carr'be-obtained. v

Feathe -purposes -.oithe invention, further,- the illuminated. strip. 14 :is madeeto approximatean' infinitely long; mathematical-line of-ihole generation. The-region over which hole-electron pairs are generatedeis; actually; a-thin, narrow-rectane gular parallelepiped of length-l, width w. and depth.d,.as depicted iii-Fig.3 Ifthe ratios of .both 111, and .d .tOitheZminimum separation r :are made 7 much :less thanaunity then .one may assume that the :origin .ofrthe holes .is a; line extending down the-middle-ofi the-strip, l 4.- If.v these ratios are kept below about one-fifth-a. good approximation can be had; Similarly, iithelengthl is greater than about five times .the: maximum :separator r arr-infinitely long line isadequately-approximated. If now r is varied over avrangeof values, the i measured voltage-V acrossresistor. I I due to the difiusing holes'may be found to vary'. with 1 in the manner represented, for-specific example, by curve A'inFig'G; Inthelatter; the 'logarithm of the ratio'of V- to the. measuredzvoltage V0 at an arbitrarily-selected reference value of r is plotted againstr: In thecurve'n so plotted the portion tottheleft, corresponding to:sinall values .of r, is not of fpresentslgnificance :for itireflects ;distor-.-

and the finite width of the illuminated area. This last effect of finite line width was discussed above. Consider now the distortion resulting from the collector field. This distortion is negligible for values of r greater than about where p is the resistivity (in ohm-centimeters) of the semiconducting material and I is the steady biasing current (in amperes) flowing through the collector. This limiting value of r is about0.0l2 inch in a representative case where the resistivity is 5 ohm-centimeters and the collector current 1 milliampere.

Excepting for the above-identified portion, at sufliciently large values of r, the slope m of the curve A, indicated by curve T, is substantially uniform. We have found that the slope m affords a sensitive index to the intrinsic lifetime T17 of the test piece, as will be evident from Fig. 6' where curves are shown for test pieces having various intrinsic lifetimes. Thus, curve A applies to a test piece that was found by prior art methods to have a Tp of 200 microseconds and curves B, C, D and E indicate the corresponding variations for test pieces having Tp values of 5', 10, 50 and 400 microseconds, respectively.

In the practice of th invention, therefore, the slope of the measured characteristic of any particular test piece may be compared with the corresponding slopes of a number of standard pieces measured in the same way and under the same conditions. The standard pieces may have known values of Tp, in which case Tp for the particular test piece may be determined by interpolation.

Alternatively one may compare the measured charact ristic of the test piece, plotted as in Fig. 6, with a family of similar plots, for different values of the diffusion length, or life path, 11p, of the zero order Hankel function of pure imaginary argument p (r) as follows:

where m is a constant corresponding to the value of p at some arbitrary fixed value of r=ro. The measured curve A in Fig. 6 has a slope, at the larger values of r, that is best fitted by the Hankel function curve corresponding to a diffusion length of Le=0.094 centimeter, the latter being shown at H in Fig. 6. Since Tp L p/Dp, where Dp is the appropriate diffusion constant, 'rp can now be calculated. For example, if the test piece is n-type germanium, as assumed, having a diffusion constant Dp for holes of 44 cm. /sec. at room temperature, the calculated lifetime is 200 microseconds. If the measured data pertained instead to p-type germanium having a diffusion constant D of 92 cmP/sec. the calculated lifetime would be 96 microseconds. It should be appreciated that no significance is to be attached to the vertical fit of the curves A and H in Fig. 6 inasmuch as both curves are essentially plots of ratios.

The alternative method last described can readily be extended to cases in which the line width w is not negligible by integrating the expression for 29(1') over the width to and using the resultant in lieu of curve H.

The significance of surface recombination velocity, and its reduction, in the practice of the present invention can now be examined more critically and better understood. In the limiting case, the surface recombination velocity s would be equal to the radial, or volume,

diffusion velocity, the latter being given to a good approximation by D/L= /D/1-. For the purposes of the invention the value of D/ should be made substantially larger than the values of Tr) to be measured. Useful results may be obtained if D/ is only twice 7' but if D/s is made at least four times 'Tp a much higher degree of accuracy can be achieved. Suppose, for specific example, that the test pieces are of n-type germanium for which D is 44 cmF/sec. and that Tp is of the order of microseconds. If the surfaces are treated to reduc s to about 200 om./sec., IJ/s will be about 1100 microseconds or about seven times 'Tp, and this will permit accurate measurements to be obtained over a considerable range of values of T1).

Further, as L /s is progressively reduced below Tp, the plotted curve A exhibits a concave-downward curvature, even for the larger values of T, that makes it difiicult to fit it to a Hankel function curve such as H. In such cases, it is best to compare the data to curves obtained in the same manner on standard pieces whose lifetime has been determined by other methods; see, for example, J. R. Haynes and W. Shockley, Phys.

EeV., V01. 75, page 691 (1949) amplifier 2| to a linear rectifier 22 that is fol-- lowed by a low-pass filter 23. The latter has a cut-off frequency fc such that w2 fc w1, where m is the light-chopping frequency, car is the carriage-moving frequency, and w2 w1. The rectifled output of filter 23, fluctuating with the movement of carriage ll, is applied to the pair of deflecting plates of a cathode ray tube 24 that controls deflection in the vertical plane, while to the other pair of deflecting plates is applied a sweep voltage wave that is synchronized with cyclical movement of the light beam across surface 7. The latter movement may be effected, as illustrated diagrammatically, by a rotating cam 25 that displaces the carriage l! in one direction against a spring that returns the carriage and keeps it in contact with the cam. Mechanically linked with the cam is the rotatable arm of a potential divider 26 which supplies to the other pair of deflecting plates a sweep voltage that varies linearly with the displacement of the carriage from one of its extreme positions. The screen of the cathode ray tube may be of the long persistence type, and the frequency of the sweep wave, and of the movement of the carriage, may be as low as desired as long as a readable trace is obtained on the screen, e. g. 5 to 10 cycles per second.

The horizontal dimension of the screen has the sense of a linear scale of distance 1. Since the output voltage of amplifier 2! varies as the logarithm of its input voltage, the vertical dimension of the screen has the sense of a logarithmic scale of voltage. In operation, therefore, there appears on the screen a curve that is of the same character as curve A of Fig. 6. The slope of the curve-so appearing varies from one test piece to another in correlation withthe variations in in trineicclifetimec Calibrating dines; corresponding;- te'the'curves.=B;C-, D etc. of Fig; 6, inay-bezmarked;

on" the screen .to facilitate ideterminationtof relative flifetime and zof absolute lifetime'also if desired.

' What'islclaimed is:

1:. The'zmethod of'licompa ring b'odiessof semiconductive material with respect to intrinsic lifetime .whichcomprises forming a-srnooth. surface.

on'each'of saidibodies, treating-said surfaces alike toareduce substantiallythe :recombination veloci ties. thereof ,i directingi a beam .of radiant energy onneach saidisurface tocgenerate mobile'charge' carriersi irfa .striplilcef portion thereof; detecting thew concentration diffusing mobile charge carriersin each saidbody at'afixed lace onsaid surface separated; fromisaid sti'ipdikeportion' by an extended; substantially field-free region, concurrently varying the. separation of said striplikeportion from sa-id'fixed placewhilemaintainingzeachtsaidbody:atsubstantially the same predetermined-temperature; and'indicatin'g' the rate 1 atavhich saiddetectedconcentration varies with saidivariation in separation:

2. The method of comparing bodies of semiconductive material with respect tointrinsic lifetime of :mobile: charge-carriers which comprises formingaismooth surface on eachof said bodies, treating-said surfacesialikex to provide a surface recombinationvelocity that is small compared .to the diffusionvelocity, illuminating each of said surfaces at a. distance from a fixed reference pointthereonyselectively detecting the density at saidireferencezpoints of'the mobile charge carriers generated in said bodies bythe illumination of said treatedsurfaces and diffusing to said reference points; and. determiningzthe relative rates of? change in the detected. density with variation of saiddis'tan'ce while maintaining said bodies at thesame temperature.

3: Thermethodflof testing bodies of Jsemiconductiveimaterial with respect'to relative intrinsic" lifetimerofmobilecharge carriers which comprises forming a:smooth surface oneach of said bodies; treatingeachof said surfaces'alike toreduceethe-recombinationvelocities thereof to a value s'sucli'cthattD/s is substantially greater than: the range of intrinsic lifetimes of said bodies, whereD is the diffusion constant applicable to saidicarriers, directing "a beam of light against each: said surface-to illuminateaportiorr thereof, measuring at substantially the same'temperature therconcentration of said carriers: in each said bcdyzat fixed place on said surface separated from said illuminated portion and to'which placesaid carriers are free. to move substantially solely by-diffusion, and varying the separation of said fixed aplace, an electrical indicatorconnected to' said' electrodes,zand a logarithmic transducer electrically;interposecliirr. thevconnection'ibetween sai'dielectrodes and said. indicator.

5-; .A device: of the: kind r described comprlslnzr means zfor holding-a body of semiconductive' ma terial tobe: tested, means 'to direct a beam of light, modulated at a first'cyclical rate, toilluminate a strip-like pjortionof a surface'of thebody so held, a collector.electrode adapted to make contact with said'surface at a fixed point thereon, means to vary the position of said illuminatedtportion relative to said fixed point-at a secondcyclical rate substantially lower'than said first cyclical rate, a second electrode adapted' to makeicontact withsaid body, an electricalcircuit including a currentsource connecting said electrodes, a logarithmic 'transducer'the input of."

first raydeflectorxoperatively connected to r the output of said filter means" and. a v second: rayi deflectoroperatively connected to said sweep voltage source.

'7. 'I'hemethod of'testing bodies of a"semicon ductive material selected from the class iconsisting' of germanium'an'd silicon with: respect to relative intrinsic lifetime of mobile charge car riers, which comprises: forming onxsaid: bodiesrespective smooth surfaces, treating said surfaces alike to impart .a surface recombinationvelocity s such that D/s differs by at least several fold T.

from the range of intrinsic lifetimes: of said bodies; where D is the diffusion constanttapplicable to saidcarriers, directing a'b'eam offradiant:

electromagnetic energy againsta-tportion-ofeach said surface to generate mobile charge carriers in the body, measuring at substantially the same temperature the concentration of 'carriersin'each said body at a fixed place on said surface sepa- .rated from said surface portion by a substantially field-free region, and; concurrently with said measurement'of concentration, varying the posi-- tion of said'beam to vary the extent of said separation.

8. The method-of testing, for relative intrinsic lifetime of mobile charge carriers, masses of semiconductivematerial whosedimensions are all of the order of centimeters which method comprises forming 'a smooth face on each said mass, treat- ,ing'sai-d smooth faces alike to reduce the recombin'ation'velocities thereof to a value s such that D/s is several times 'less than the range of intrinsic lifetimes of 'said masses, where D is the di'ifusion'constant applicable to said carriers, to said carriers, directing a beam of radiant energy to'impinge on a portion of each said face that overlies asubstantially field-free region in the mass and'ithat is remote from other surfaces thereof whereby said impinging beam generates mobile charge carriers that are free to diffuse uniformly in' a pattern dependent substantially only on'intrinsic characteristics of 'the material and said face, measuring at substantially the same temperature the concentration of said diffusing generatedcarriers in'each said mass at a fixedplace on said face, and concurrently with the measurement of concentration shifting the place of impingementof .said beam, withinsaid face portion, relative? to said fixed place i whereby; the relative. intrinsic lifetimesi charactteristic of said masses is readily determinable from the respective variations in measured concentration with the separation of said fixed and shifting places.

9. In the fabrication of semiconductor translating devices from ingots of material selected from the class consisting of germanium and silicon, the method of comparison testing said ingots with respect to intrinsic lifetime of mobile charge carriers which comprises forming on each said ingot a smooth fiat surface, treating said surfaces alike to impart a surface recombination velocity s such that D/s differs by at least several fold from the range of intrinsic lifetimes of said bodies, where D is the diffusion constant applicable to said carriers, directing a beam of light of periodically varying intensity against each said surface to generate mobile charge carriers in a body portion of the same linear strip-like shape, selectively measuring at the same temperature the relative concentration 10 of said light-generated carriers in each said body at a fixed place on said surface to one side of said linear strip-like portion and separated therefrom by an extended substantially field-free region, and, concurrently with said measurement of concentration, shifting the position of said beam and of said strip-like portion within said substantially field-free region to vary the extent of said separation, whereby the relative intrinsic lifetimes characteristic of said ingots are uniquely correlated with the respective rates of variation of measured concentration with separation.

References Cited in the file of this patent Article by T. W. Case in Physical Review, Series II, v01. IX, 1917, pages 305-309.

High Frequency Operation of Transistors, by C. B. Brown, Electronics, July 1950, vol. 23, pages 81-83.

,The Phototransistor, by J. N. Shive. Bell Laboratories Record, August 1950, pp. 337-342. 

